1. Field of the Invention
This invention relates to a recording apparatus employing an electrophotography system such as printer, copying machine, facsimile or the like, and is particularly concerned with a fixing apparatus for fixing a toner image to a recording medium where the image is formed on a surface of the recording medium.
2. Description of the Prior Art
A prior art fixing system has at least a heat roll and a back-up roll. In this specification, there may be a case where the heat roll is abbreviated as HR, and the back-up roll as BR. Then, both the rolls HR and BR may be called fixing rollers coupled to one another. In the fixing system, HR is heated, and HR and BR rotate as coming in contact with each other. The drive roll in this case is either one or both of HR and BR. Both HR and BR coming in contact with each other may form a contact face. The contact face is then called a nip zone. A recording paper having a toner image not yet fixed passes the nip zone as nipped between HR and BR. In this case, the unfixed toner is heated by HR to melt. The melted toner passes the nip zone and then solidifies to fix on the recording paper. A series of such operations is called fixation.
Generally in the fixing system, it is preferable that the toner, after fixation, sticks to the recording paper satisfactorily hard, and even in case the image is rubbed or bent after fixation, the toner will never come off. In this specification, such sticking strength is called fixing strength.
However, in the prior art fixing system, when the fixing roller couple rotate at high speed, it is difficult to obtain a satisfactory fixing strength only through the fixing roller couple. In this case, the fixing strength will be increased simply from enhancing a meltability of the toner without applying an improvement to the fixing system particularly therefor. However, enhancing a meltability of the toner can shorten a term of preservation. A stability of development deteriorates at the same time, a fog arises on an image, and where striking the toner blocks within a developing machine, thus coming to unprintableness. Further, the toner scatters externally of the developing machine, and adheres to a conveyor and other elements to cause an erroneous printing on the recording paper. Where the toner strikes, a coefficient of friction of the conveyor drops. This leads to an incapability of conveyance, thus exerting an influence on the system entirely. Accordingly, it is necessary to obtain a satisfactory fixing strength under the toner condition that such disadvantage will never be caused. Thus, as mentioned in Photographic Science and Engineering 27, (1983) pp 19-25, there may be a case where a preheater for heating a recording paper and an unfixed toner supplementarily from behind a printing surface of the recording paper is provided immediately ahead of the fixing roller couple in the direction where the recording paper is carried.
It is then most general that HR comprises a cylinder having a structure with the surface of a mandrel which is a metallic hollow cylinder coated with a nonviscous heat-resisting material such as fluororesin or silicone rubber, and that of having a structure with a heating element such as halogen lamp or the like provided centrally of the cylinder. In this specification, the nonviscous coat is called a surface layer. The surface layer is provided so as to minimize an occurrence of a toner offset phenomenon wherein a toner adheres to the HR surface when the toner passes the nip zone. The heating element operates according to a detection signal of a heat sensor for detecting temperature of the HR surface or thereabout.
Further, the following is disclosed in Japanese Patent Unexamined Publication No. 57-100459. The fluororesin is a bad heat conductor, therefore it is desirable that a membrane thickness of the fluororesin be minimized so as to reduce the time for the HR surface to reach a predetermined temperature after the heating element is actuated. It is advisable that the membrane thickness be 20 .mu.m or below in a distance from the nose of a protrusion of ruggedness of a membrane inner layer of the fluororesin to the surface of the membrane.
Further, the following is mentioned in Japanese Patent Unexamined Publication No. 60-213974. That is, since a heat conductivity of a heat-resisting resin of the surface layer is unsatisfactory, it is difficult to transfer a heat energy from the heating element. Thus the HR surface, a heat response efficiency is weak and furthermore a heat loss is considerably extensive. When a membrane thickness of the surface layer is 40 .mu.m to 50 .mu.m, a difference in temperature between the heating element and the HR surface reaches 20.degree. C. to 25.degree. C. Accordingly, when much heat is taken away suddenly from the HR surface through copying incessantly, a heat supply from the heating element to the surface layer becomes insufficient temporarily, and thus HR peripheral temperature drops to cause a defective fixation. Further, in case much heat is taken away from a central portion of the HR surface and thus the surface temperature of the central portion drops temporarily through copying continuously on a narrow and small-sized recording paper, temperature of the heating element rises according to an action of the heat sensor and a heat supply to the HR peripheral surface is increased, therefore the surface temperature considerably rises on opposite end sides of HR. Accordingly, the surface temperature exceeds a heat-resisting temperature of the heat-resisting resin layer to shorten the lifetime, or when copying on a broad and large-sized recording paper after copying continuously on a small-sized one, a toner offset phenomenon arises on opposite end sides of the paper.
Further, Japanese Patent Unexamined Publication No. 56-159676 may be taken up as referring to a prior art example. A thickness of the surface layer has been specified at 30 .mu.m or below. Then, from specifying the thickness of the surface layer at 30 .mu.m or below, a heat supply is made from a roller mandrel at the nip zone, and as a result, temperature of the surface layer rises.
An apparatus of the aforementioned prior art which uses the preheater and rotates the fixing roller couple at high speed to fixation involves the problem that a large-sized construction is quite unavoidable with a fixing system comprising the preheater. Further, since the preheater must heat the recording paper from a non-print surface, in the case of high-speed double-side printing recording system where the non-print surface may not be present, the preheater cannot be provided. Consequently, when the fixing system is used in the state where the high-speed printing recording system is not provided with the preheater, a satisfactory fixing strength will not be obtainable.
Still further, a prior art in which the surface layer consisting of fluororesin is thinned in thickness without using the preheater is not so problematical when the number of sheets to print is small, however, when printing so much continuously, a quantity of heat moving from a mandrel on the lower layer to the surface layer increases from the surface layer having been thinned in thickness. Thus, when carrying out a fixation continuously, temperature of the HR mandrel suddenly drops, the HR surface temperature also drops correspondingly thereto, and a satisfactory fixing strength is not obtainable after all.
Meanwhile, a toner remains partly as a stain on the heat roll at the time of fixing operation. Consequently, a cleaning device for removing the stain is normally provided on the heat roll. Japanese Utility Model Publication No. 63-50669 may be taken up as referring to such prior art. The prior art then comprises a construction to wipe out a stain on the heat roll as moving a wiping belt from a let-off roller toward a take-up roller in the counter direction to that in which the heat roll rotates. The wiping belt consists of a felt 1 mm or so ordinarily in thickness. Further, Japanese Patent Unexamined Publication No. 57-118283 may be taken up as giving another prior art. This prior art comprises a construction wherein a wiping member is disposed in contact longitudinally of the rotating heat roll, wiping out a toner remaining on the heat roll. Then, the wiping member is formed of a coarse nonwoven fabric such as felt 3.2 to 12.7 mm thick or the like.
The aforementioned take-up type cleaning device is 1 mm thick, therefore the wiping belt is taken up so quickly, and hence the replacing work is required frequently. For the coarse construction, the cleaning device using a felt 3.2 to 12.7 mm in thickness has a disadvantage that a toner moved from the heat roll to the wiping member is capable of adhering again to the heat roll. Such problem is inherent not only in the cleaning device provided on the heat roll but also in a cleaning device provided on a back-up roll of the recording apparatus and a photosensitive member.